Exhaust gases from internal combustion engines contain hydrocarbons (HC) stemming from the incomplete combustion of the fuel in the engine. In order to bring about a post-engine conversion of such unburned hydrocarbons, Otto combustion methods as well as diesel combustion methods make use of exhaust gas catalysts that oxidize the hydrocarbons in the presence of oxygen in order to form carbon dioxide and water. Aside from oxidizing unburned hydrocarbons, such catalysts—which are also referred to as oxidation catalysts—also oxidize carbon monoxide to form carbon dioxide. If the catalyst is not only capable of performing oxidation but also of reducing nitrogen oxides present in the exhaust gas, one refers to them as three-way catalysts since they convert all of the exhaust gas components (HC, CO and NOx) that fall under statutory limit values.
Since the HC-conversion efficiency of oxidation or three-way catalysts can deteriorate as they age, it is desirable to carry out a diagnosis of the catalyst in order to determine its HC-conversion efficiency. For this reason, various laws regulating the emission limit values for motor vehicles stipulate that catalysts must be monitored within the scope of on-board diagnostics (OBD). If the HC-conversion efficiency of an aged catalyst has deteriorated to such an extent that the vehicle exceeds certain emission limit values, the catalyst has to be recognized by the engine controls as being defective and this has to be indicated in a suitable manner.
For purposes of measuring the HC-conversion efficiency of a catalyst, it is a known procedure, for example, to evaluate the exothermic characteristics of the catalytic reaction of the HC conversion. For instance, late post-injection of fuel artificially increases the HC fraction in the exhaust gas from the internal combustion engine, and then the temperature of the catalyst is measured by means of a temperature sensor located downstream from the catalyst. In this context, owing to the exothermic characteristics of the HC combustion that takes place in the catalyst, a large temperature increase correlates with a high HC-conversion rate. Conversely, an aged oxidation catalyst can be recognized on the basis of a small temperature increase. In order to nevertheless achieve a significant temperature increase, a high HC fraction in the exhaust gas is necessary, that is to say, a large quantity of post-injected fuel. This, in turn, raises fuel consumption as well as the final emissions of hydrocarbons.
Another known method employs the oxygen storage capacity (OSC) of oxidation catalysts. Many oxidation catalysts, especially in the case of Otto engines, exhibit a high capacity for oxygen storage due to the use of suitable additives such as, for example, cerium oxide. A widespread diagnostic approach involves the measurement of the oxygen storage capacity of the catalyst by means of two lambda probes that measure the oxygen content of the exhaust gas before and after the catalyst. Here, the engine is at first run for a prolonged period of time at a rich air-fuel ratio, so that the exhaust gas has a low residual content of oxygen and a high HC fraction, as a result of which the catalyst is freed almost completely of oxygen. Subsequently, the engine is run at a lean ratio, so that its exhaust gas has a high residual content of oxygen and a low HC content at the engine outlet. The transition from rich to lean in an intact oxidation catalyst is measured by the downstream lambda probe with a pronounced time delay since the oxygen at first fills up the OSC of the catalyst and can only subsequently be measured at the catalyst outlet. As the catalyst ages over the course of time, the transition from rich to lean is measured sooner and sooner since the OSC is reduced due to ageing or due to a defect. Fundamentally, the diagnosis can also be carried out after an abrupt change from lean to rich. The drawback of this approach is that it is not the HC-conversion efficiency itself that is measured, but rather the oxygen storage capacity of the catalyst. In various operating states, however, these two variables only correlate to a certain extent. Moreover, the diagnosis of the oxygen storage capacity is greatly influenced by dynamic effects and consequently not very reliable during operation.
German patent application no. DE 10 2007 012 701 A1 discloses the technique of monitoring the function of an oxidation catalyst by means of a particle sensor installed downstream. A resistive particle sensor is employed which detects deposited particles by measuring the electric resistance between two interdigital electrodes. Such sensors have a cross sensitivity to unburned hydrocarbons since the latter are deposited on the electrodes and likewise cause an increase in the resistance. The method now calls for introducing systematically increased HC fractions into the exhaust gas upstream from the catalyst and for observing the response of the particle sensor downstream from the catalyst. An increasing sensor current will be measured if the catalyst is defective.
German patent application no. DE 10 2005 056 312 A1, in contrast, utilizes the HC cross sensitivity of a broadband lambda probe installed downstream from the oxidation catalyst in order to monitor it in terms of its HC-conversion efficiency. Owing to the HC cross sensitivity of the probe, the probe signal deviates from the true signal value that corresponds to the actual oxygen content. For this reason, it is proposed that a determination be made of the amount by which the measured probe signal of the broadband lambda probe deviates from the actual lambda value. In an intact oxidation catalyst, the deviation is close to zero, whereas the deviation is large if the catalyst is defective. The lambda actual value is either ascertained with a sensor that does not exhibit any HC cross sensitivity or else it is calculated on the basis of models.
German patent application no. DE 10 2008 041 385 A1 makes use of the HC cross sensitivity of a broadband lambda probe located downstream from the oxidation catalyst in order to recognize a defective catalyst. In this context, however, the HC cross sensitivity is used to detect a shift in the light-off temperature of the catalyst as it progressively ages. Here, the broadband lambda signal is detected downstream from the catalyst during the warm-up. Relatively high HC fractions are encountered downstream from the catalyst when it is still cold because its activity has not yet started. These high fractions account for a shift of the displayed signal value away from the true signal value that corresponds to the actual air-fuel ratio. As the catalyst heats up and the HC conversion starts, the HC fraction downstream from the catalyst decreases and thus so does the signal shift caused by the HC cross sensitivity. In the case of an intact catalyst, which has a comparable light-off temperature, the signal shift starts correspondingly sooner. A very aged catalyst, in contrast, can be recognized on the basis of the start of the signal shift that is late relative to the “true” signal value.
Before this backdrop, the present invention is based on the objective of putting forward a method to determine the HC-conversion efficiency of a catalyst, whereby this method generates relatively few process-related HC emissions, exhibits a high degree of reliability and independence from the operating point, and can be realized to the greatest extent possible without additional structural measures.